This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to an arrangement for preventing the buildup of electrostatic charge on the display screen of a CRT by directing the charge to neutral ground.
CRTs operate at high voltages causing the glass display screen, or faceplate, surface of the CRT to become electrically charged by static induction. The static electricity attracts dust and other contaminates in the air causing them to collect on the display screen""s outer surface which degrades the video image presented on the display screen. When a person touches the CRT""s display screen, he or she may receive a slight shock by discharge of the static electricity, particularly under relatively low humidity conditions. In addition to being unpleasant to the touch, this static discharge may disrupt the operation of other electronic equipment located nearby such as a computer when the CRT is employed in a computer terminal. Other types of self-emitting video displays such as liquid crystal displays (LCDs), plasma discharge displays (PDPs), vacuum flourescent screens, and gas discharge screens also suffer from the aforementioned problems. The buildup of static charge on the faceplate of a video display arises because of the dielectric nature of glass. In addressing problems arising from static electricity buildup on the display screen, an electrically conductive antistatic coating is typically applied to the outer surface of the display screen and is coupled to neutral ground for dissipating static charge on the display screen""s outer surface.
The outer surface of the CRT""s display screen reflects approximately 4-8% of the light incident on the display screen. This reflected light not only degrades video image resolution and contrast, but also causes viewer eye fatigue. To reduce this light reflection and improve video image viewing, an outer antireflective layer, or layers, is deposited on the display screen""s inner antistatic layer. In some cases, the antireflective and antistatic layers are mixed together to form a single solution which is applied to the display screen""s outer surface as a single layer coating. Even when the antistatic and antireflective layers are sequentially deposited on the display screen, there is to some extent fusion between the two layers, particularly when the layers are applied by a wet coating method such as by spray or spin coating.
To avoid the problems of static charge buildup on the display screen, the display screen""s conductive antistatic layer is typically connected to neutral ground for dissipating this charge. One approach to grounding the display screen""s outer surface coating using the CRT""s implosion protection, or tension band is disclosed in U.S. Pat. No. 5,025,490 and involves the application of an electrical conductive tape to the outer layer of the 2-layer coating and attaching the conductive tape to the grounded implosion protection band. The electrically conductive tape used in this application is rather expensive and cannot be used where the antireflective layer is separate from and covers the antistatic layer such as when the coating is applied by sputtering.
Referring to FIG. 1, there is shown a typical prior art installation for electrically grounding the glass display screen 14 of a CRT 10. In the following discussion, common elements in the various arrangements are described with the same terminology and have assigned the same element identifying number. In addition, the terms xe2x80x9cdisplay screen,xe2x80x9d xe2x80x9cdisplay panelxe2x80x9d and xe2x80x9cfaceplatexe2x80x9d are used interchangeably as are the terms xe2x80x9clayerxe2x80x9d and xe2x80x9ccoatingxe2x80x9d. CRT 10 includes a sealed glass envelope 12 having a rear neck portion 18, an intermediate funnel portion 16, and the aforementioned glass display screen 14 on a forward portion thereof. Disposed within the CRT 10 are one or more electron guns (not shown for simplicity), each of which directs a respective electron bean on the inner surface of the glass display screen 14. Disposed on the inner surface of the glass display screen 14 is a phosphor screen (also not shown) which includes plural discreet phosphor deposits, or elements, which emit light when an electron beam is incident thereon to produce a video image on the display screen. Disposed about the outer periphery of the sealed glass envelope 12 is an implosion protection, or tension, band 70 typically comprised of a high strength, electrically conductive material such as steel. Attached to respective corners of the outer periphery of the implosion protection band 70 are first, second, third, and fourth band ears 28a-28d. Each of the band ears 28a-28d is securely attached to the outer surface of the implosion protection band 70 by conventional means such as weldments. The band ears 28a-28d are used for mounting the CRT 10 in a chassis which is not shown in the figure. Disposed on the outer surface of the glass display screen 14 is a composite antistatic/antireflective coating 52. The composite antistatic/antireflective coating 52 may be in the form of a single layer or in the form of an inner antistatic layer applied directly to the glass display screen 14 and an outer antireflective layer applied over the inner antistatic layer. The composite antistatic/antireflective coating 52 is electrically conductive, as is the antistatic layer in the case of the two-layer coating.
FIG. 2 is a simplified sectional view of a corner of the CRT""s sealed glass envelope 12 showing a single composite antistatic/antireflective coating 52 such as produced by a wet coating application, i.e., a spray or spin coating, on the outer surface of the CRT""s glass display screen 14. In the wet coating process, there is typically diffusion between the antistatic and antireflective layers to produce a composite coating comprised of molecules 51 arranged in a staggered array. In the two-layer arrangement as shown in FIG. 3, an inner antistatic layer 46 is maintained separate from an outer antireflective layer 48 on the outer surface of the CRT""s glass display screen 14. The separate inner antistatic layer 46 and outer antireflective layer 48 are typically applied using a dry coating method such as by sputtering. In both cases, the prior approach involves grounding the antistatic/antireflective coating by electrically coupling it to the CRT""s implosion protection band 70 which is connected to neutral ground by means of a grounded connection 44 (shown in dotted line form). In the case of the composite antistatic/antireflective coating 52 shown in FIG. 2, an edge of the outer surface of the composite antistatic/antireflective coating is electrically coupled to the implosion protection band 70 by means of a strip of conductive aluminum tape 50. Applied over the conductive aluminum tape 50 is a plastic film of conductive aluminum foil 60 having an adhesive backing 60b for adhesion to the outer surface of the conductive aluminum tape. This arrangement is also shown in the perspective view of FIG. 1. In the case of the two-layer arrangement comprising the inner antistatic layer 46 and the outer antireflective layer 48, the conductive aluminum tape 50 is applied to an outer edge of the outer antireflective layer as well as the CRT""s implosion protection band 70. A plastic film of conductive aluminum foil 60 is placed over the conductive aluminum tape 50 as in the previously described arrangement. Because the outer antireflective layer 48 is not as good a conductor as the inner antistatic layer 46, the grounding arrangement shown in FIG. 3 is not as effective in grounding an electrostatic charge which may be present on the display screen as the arrangement shown in FIG. 2 for the composite antistatic/antireflective coating 52. In order to improve the grounding capability of the arrangement shown in FIG. 3, a conductive element 55 is sometimes formed on the outer surface of the inner antistatic layer 46 so as to extend through the outer antireflective layer 48 as shown in FIG. 4. Conductive element 55 reduces the electrical resistance between the antistatic layer 46 and the conductive aluminum tape 50. Conductive element 55 is typically comprised of a conductive metal and is formed by conventional means such as ultra-sonic spot welding.
The conductive aluminum tape used in the grounding arrangements described above and shown in FIGS. 1-4 is relatively expensive and thus increases the manufacturing cost of the CRT. In addition, the conductive aluminum tape is typically applied by hand by a worker which further increases CRT manufacturing cost.
The present invention addresses the aforementioned limitations of the prior art by providing for the electrical grounding of an antistatic/antireflective coating on the outer surface of a CRT""s display screen by applying a layer of conductive carbon black between the antistatic/antireflective coating and the CRT""s grounded implosion protection band. The inventive electrical grounding of the antistatic/antireflective coating is applicable to either a single composite antistatic/antireflective coating applied by a wet coating process as well as to the combination of an inner antistatic layer and an outer antireflective layer such as applied by a dry coating process in providing a highly reliable, low cost approach to static charge dissipation on the CRT""s display screen.
Accordingly, it is an object of the present invention to direct electrostatic charge on the outer surface of a display screen of a CRT to neutral ground.
It is another object of the present invention to increase CRT viewer safety by preventing electric shock to the viewer and to reduce electrostatic interference with the operation of other electronic devices caused by electrostatic charge buildup on the CRT""s display screen.
Yet another object of the present invention is to improve viewing of a video image on the display screen of a CRT by reducing the deposit of dust particles and other contaminants on the screen due to the presence of an electrostatic charge on the screen.
A still further object of the present invention is to provide an arrangement for grounding an antistatic/antireflective coating applied to the outer surface of a CRT display screen by spray coating, spin coating or sputtering.
These objects of the present invention are achieved and the disadvantages of the prior art are overcome in a CRT having a sealed glass envelope with a glass display screen whereon is presented a video image produced by plural electron beams incident upon a light emitting coating disposed on an inner surface of said glass display screen, and wherein an electrostatic charge is produced on the display screen by the electron beams incident thereon, by an arrangement for dissipating the electrostatic charge comprising a grounded implosion protection band disposed about and engaging an outer surface of the sealed glass envelope; an electrically conductive antistatic/antireflective coating disposed on an outer surface of the glass display screen for receiving an electrostatic charge on and reducing light reflection from the glass display screen; and a conductive carbon black layer disposed on a corner of the glass envelope and coupling the antistatic/antireflective coating to the implosion protection band for directing an electrostatic charge on the display screen to neutral ground.